APLICACIÓN DEL MÉTODO EXPERIMENTAL AL ESTUDIO DE LA DENSIDAD Conocimientos previos: ¿Qué es materia? ¿De qué está formada la materia? ¿Qué es la masa? ¿Qué es el volumen? ¿Cómo podemos medir la masa? ¿Y el volumen? Formular hipótesis : haciendo predicciones. Diseñar y llevar a cabo EXPERIMENTOS Analizar el resultado de los experimentos: Modelos científicos. Sacar una conclusión
Observación de un hecho cotidiano:
Comparamos una bola de gran volumen pero con muy
poca masa con una piedra mucho más pequeña y
con una gran masa.
Comportamiento del aceite al mezclarlo con agua.
What is matter? But, what is mass? What do we call the space that an object takes up? What is matter made of? It’s everything that takes up space and has mass. It’s the amount of matter in an object. Volume Atoms Repasamos los conocimientos previos
Look at the stone and the ball and… pick them up. Which object has more volume, the stone or the ball? And… which one has more mass? Presentamos un hecho cotidiano The ball. The stone.
How is this possible? How can we explain this? OK, take the ball and knock on it. Do you think it’s a hollow ball? Any other ideas? Don’t forget what matter is made of. Perfect. But not only that, are the two objects made up of the same material? Because the white ball is empty inside. No, it isn’t. I know. The atoms in the stone are closer than in the ball. No, they are made up of different materials.
OK. Let’s pretend that we are atoms. Look at this square. Please, all of you, enter the square. Imagine you are the atoms of one of the objects. Now, half of you step out of the square, and those of you inside take up all the space. Now you are atoms of the other object. Which square represents the atoms in the stone? I think the first one, because the atoms inside the stone are much closer.
That’s right. Let’s do it again. I’m going to say: ‘stone’ or ‘ball’ and you have to behave like the atoms of the object I’m saying. Foto grupo Do both squares take up the same space? So, do they have the same volume? Do they have the same mass? Analizamos lo que hemos observado Yes, they do. Yes. No. The square that represents the stone has more mass than the square that represents the ball.
Sacamos conclusiones What have you seen? So, what is your conclusion? Fantastic! This is what we call DENSITY Conclusion: DENSITY is the relationship between mass and volume. (m/V) Both squares take up the same space but the square that represents the atoms in the stone has more mass. Some materials have more mass than others when they have the same volume.
Let’s see how can we measure the density of these two objects. Repasamos los conocimientos previos If density is the relationship between mass and volume, what do we need to know to measure the density? How can we measure the mass? And what are the units for mass? How can we measure the volume? What are the units of volume? The mass and the volume. With a balance or some scales. Grams and kilograms. With a measuring jar. Mililitres and litres
Let’s do it. Experimentamos
Aprendemos a calcular la densidad todos juntos utilizando la pizarra digital. Now, let’s calculate the density of this stone and this ball.
Repasamos en casa los conceptos aprendidos en la clase .
We are going to compare the density of same materials with water. What happened when you put the stone into water? Did it sink or float? It sank. Can you explain why? Because it is heavier than water. Presentamos un hecho cotidiano And what about the ball, did it sink or float? It floated
Formulamos hipótesis Think about densities, has water more or less density than the stone? Another question: has water more or less density than the ball? I think water has less density than the stone. I think water has more density than the ball.
Let’s check it. Look at this bottle full of water. Can you tell us the volume of water? We know the volume of water, but what else do we need to calculate the density? Experimentamos 330 ml. The mass of water inside the bottle. The mass of water is 341 gr.
Let’s calculate the density of water. Density= mass/ volume Density of water= 329 gr / 330 ml= 0·99 gr/ml There is a mistake in our measurings, because the mass we have measured is not only the mass of the water, it was the mass of the water and the mass of the plastic bottle. So, we can say that the density of water is 1gr/ml Now we have to measure the mass of the empty bottle. Mass of water = mass of full bottle – mass of empty bottle Mass of water = 341 gr – 12 gr = 329 gr
Analizamos los resultados Were your predictions right or wrong? Sacamos conclusiones Do objects denser than water sink or float? Do objects less dense than water sink or float? Conclusion: Objects denser than water sink and objects less dense than water float. Right They sink They float 4 gr/ml 0,2 gr/ml 1 gr/ml Densities Stone Ball Water
Let’s check the density of these materials by comparing them with the density of water. Formulamos hipótesis haciendo predicciones.
Presentamos un hecho cotidiano What will happen if we mix water with oil? Foto grupo So, what can we say about the density of oil? And what about other liquids? Oil floats on water That oil is less dense than water.
We are going to compare the density of water with the density of other liquids, such as, alcohol, hydrogen peroxide, oil and washing up liquid. To distinguish the different liquids we need to colour them different colours by adding two drops of food colouring to each bottle.
Formulamos hipótesis haciendo predicciones.
Comprueban sus hipótesis/predicciones
Analizamos los resultados What have you seen? Oil and alcohol float in water. What does this mean? That they are less dense than water. And what about the hydrogen peroxide and the washing up liquid? They sink, so they are denser than water. Calculamos la densidad de los líquidos para comprobar que se ajustan a los resultados del experimento
To check that our experiment was fair, we are going to calculate the density of the different liquids. Remember, the volume is the same because the four bottles take up the same space, 330 ml. Now you have to measure the mass of each bottle.
Ficha de trabajo
Analizamos los resultados Conclusion: Each liquid has its own density. Liquids whose density is higher than 1 gr/ml sink in water. Liquids whose density is lower than 1 gr/ml float in water. Was the experiment fair? Yes, because the density of oil is 0·97 gr/ml. This is lower than the density of water and oil floats in water. The density of hydrogen peroxide is 1·07 gr/ml. This is higher than the density of water and it sinks in water. The density of alcohol is 0·88 gr/ml. This is lower than the density of water and it floats in water. The density of washing-up liquid is 1·04 gr/ml. This is higher than the density of water and it sinks in water.
Everything we have discovered about density, was discovered by a Greek man called Archimedes Archimedes, 287-212 BC, was born in Syracuse, a region in Sicily. He was an important mathematician, physicist and inventor. He lived most of his life in Syracuse, where he was a friend of the king. One story says that he was asked by King Hiero II to determine whether a crown was pure gold or was alloyed with silver.
“ Hmmm . . . the volume of my body equals the volume of water on the bathroom floor.” Archimedes was perplexed, until one day, observing the overflow of water in his bath, he suddenly realized that since gold is denser than silver, a given mass of gold represents a smaller volume than an equal mass of silver and that a given mass of gold would therefore displace less water than an equal mass of silver.
Another story says that he was killed while he was solving a problem by doing a drawing on the floor of his garden. When some Roman soldiers entered, one of the soldiers ordered him to accompany them and Archimedes refused, so the soldier killed him. Delighted at his discovery, he ran home without his clothes on, shouting "Eureka," which means "I have found it."